This invention relates generally to a catalyst and method for production of olefins by dehydrogenation of alkyl aromatics and more specifically to the composition and use of an improved dehydrogenation catalyst.
The prior art dehydrogenation catalysts used in the dehydrogenation of alkyl aromatics to alkene aromatics, for example, ethylbenzene to styrene as previously discussed, and alkanyl pyridines to alkenyl pyridines are widely known. Various catalysts, the dehydrogenation conditions and other operating data are disclosed in Pitzer, U.S. Pat. No. 2,866,790 relating to the use of a catalyst composition including potassium carbonate, chromium oxide and iron oxide. Other catalysts and procedures are also shown in Gutzeith, U.S. Pat. No. 2,408,140; Eggersten, et al, U.S. Pat. No. 2,414,585; Hills, et al, U.S. Pat. No. 3,360,579; U.S. Pat. No. 3,364,277; and U.S. Pat. No. 4,098,723.
U.S. Pat. No. 4,144,197 discloses the use of a catalyst composition including ferric oxides, potassium oxide, vanadium oxide, molybdenum oxide, chromium oxide cerium oxide, and cobalt oxide.
U.S. Pat. No. 3,904,552-O'Hara discloses the use of a catalyst containing ferric oxide, alkali metal salts, molybdenum oxide and cerium oxide.
While certain prior art references such as U.S. Pat. Nos. 3,904,552 and 2,990,432 teach use of Portland cement for promotion of structural stability, where the Portland cement includes calcium, it has been unexpectedly found that use of calcium provides a synergistic effect which enhances the catalyst performance, and particularly performance stability beyond that achieved by use of Portland Cement. It is believed that the inclusion of a calcium compound provides stabilization of the molybdenum component during high temperature calcination.
Additionally, catalysts within the scope of the present invention provide high selectivity by inclusion of cerium and molybdenum promoters which are further enhanced by the composition of the present invention.
Facilities for dehydrogenation of organic materials, particularly for the dehydrogenation of alkyl aromatics to alkenyl aromatics, are normally operated at the highest practical throughout rates to obtain optimum yield. Yield is dependent on conversion and selectivity of the catalyst.
Selectivity of the catalyst is defined as the proportion of the desired product (for example, styrene) produced to the total amount of feedstock (for example, ethylbenzene) converted. Activity or conversion is the representation of that portion of the feedstock converted both to the desired product and to by-products.
It is recognized that improvements in either selectivity or activity but particularly selectivity of a dehydrogenation catalyst can result in substantially improved operating efficiency.
It is generally very difficult to obtain a catalyst which has both high selectivity and high activity because high selectivity is generally accompanied by low conversion and vice versa. In general, higher conversion in catalytic dehydrogenation is favored by higher temperatures. However, higher temperatures generally result in increased production of by products and thus low selectivity. In the conversion of ethylbenzene to styrene, the predominant by-products are benzene and toluene. The benzene produced can be recycled for later processing but toluene cannot be easily recycled and is considered an undesirable by-product so that the ratio of benzene to toluene in the final product is another criteria of the effectiveness of the catalyst in this application.
It is well known that the activity of some dehydrogenation catalysts diminish with time. Ultimately, the activity of the catalyst is reduced to the point where the catalyst must be regenerated. The regeneration is accomplished by taking the production unit off stream and regenerating the catalyst by steaming. Regeneration is costly because of lost production time and due to the cost of the energy necessary to produce steam.
Accordingly, any increased stability of the catalyst, that is, long term use without diminished activity requiring regeneration enhances the economics of any process using a catalyst.